1. Field of the Invention
The present invention pertains to re-refining used oil and in particular, to a process for significantly decreasing acid sludge settling times in used oil refining processes.
2. Description of the Prior Art
Because of the huge volume of used oil from vehicle engines and the oil shortage, an oil recycling industry has grown up. Prior art processes for re-refining used mineral lubricating oil include the acid-clay method, extraction acid-clay method, distillation-clay method, distillation-hydrotreating method, and the distillation method. The acid-clay method is the most widely used method.
The current processes used to recover used oil involve heating the used oil to a temperature in the range of 212.degree.-720.degree. F., cooling the heated oil, adding acid to oxidize and remove the carbonaceous impurities, metal components and other oxidized materials, and then waiting a substantial period, generally two days or more for the acid sludge to settle.
During the lengthy interval in which the acid sludge settles, the acid, which had been added to the oil during the cooling step, is still present and in contact with the oil. Typically the acid is sulfuric acid. The lengthy contact with the sulfuric acid causes the oil to become dark brown and "burnt" or "charred" in color. The longer the oil is in contact with the acid, the darker the oil becomes. In addition, prolonged contact of the acid with the oil causes the resulting re-refined oil to be very acidic. An acidic oil is corrosive and cannot be used.
These prior art oil re-refining processes are very time consuming because of the extensive amount of time required to achieve complete settling of the acid sludge. As a result, oil recovery using these processes is not economical. In addition, these processes have had very limited success in producing quality lubricating oil. To achieve a high quality re-refined lubricating oil, all acid sludge must be removed from the oil and the color must be lightened for most consumers of such oil. However, high quality lubricating oils are difficult to produce through re-refining processes, because even after weeks or months have elapsed, the acid sludge does not settle, or has incompletely settled. Consequently, impurities remain suspended in the oil and the resulting oil is of low quality. Therefore, complete and rapid settling of the acid sludge is essential to achieving high quality, low cost, re-refined lubricating oil.
Failure of the prior art to develop a used oil re-refining process in which the acid sludge settles rapidly has also been aggravated by the cars which are currently being manufactured. The cars of today have smaller and more lightweight engines. These smaller engines get hotter than the larger engines of the past. This additional heat causes the oil to break down faster thereby producing carbon and carbonaceous impurities to build-up in the oil. Therefore oil companies are increasingly placing more additives, such as dispersant, detergent, viscosity improving compounds, or antisludge compounds, etc. into the oil. Over the years it has been shown that these additives have caused a gradual increase in the acid sludge settling time. Therefore, the problem of long settling times in prior art re-refining processes has grown even greater.
Thus, there has arisen a more urgent need to develop a process for recovering waste oil rapidly and economically by decreasing the acid sludge settling time.